Optical detection is the basic underlying principle of many optoelectronic systems like image sensors, optical communications, biomedical imaging, motion detection, surveillance, machine vision, etc. Applied research in optoelectronics has invested a lot towards the development of photodetectors with a wide spectral response (UV, visual and IR), as well as narrowband spectrum selective photodetectors for special applications. Spectral discrimination is required for colour-selective detection, but current commercial systems use broadband pho-todetectors combined with optical filters. This approach increases the complexity of the system and degrades the quality of colour detection. In this article, we explain briefly the basics of photodetectors and a method for tuning the spectral response to achieve filter-free, narrowband photodiodes.